1. Field of the Invention
This invention relates generally to methods and apparatus for drilling wells, particularly wells for the production of petroleum products, and more specifically concerns an actively controlled rotary steerable drilling system that can be connected directly to a rotary drill string or can be connected in a rotary drill string in assembly with a mud motor and/or thruster and/or flexible sub to enable selective decoupling of the actively controlled rotary steerable drilling system from the rotary drill string, such as for mud motor powered drilling, with or without drill string rotation, and to enable precision control of the direction of a bore being drilled by a drill bit and precision control of the rotary speed, torque and weight on bit being imparted to the drill bit. For mud motor speed and torque control, a controllable dump valve is provided in the fluid circuitry of the mud motor to controllably dump or divert a portion of the drilling fluid flow from the fluid circuit of the mud motor to the annulus or to bypass a portion of the drilling fluid flow past the rotor of the mud motor. This mud motor dump or bypass control valve can be automatically operated responsive to sensor signals from the rotary steerable drilling system or can be operated responsive to signals from the surface or both. For controlling weight on bit a drilling fluid powered thruster is provided in the drill string and is located above or below the rotary steerable drilling system. The thruster has a similarly controllable dump or bypass valve in its drilling fluid circuitry which is selectively adjustable by the control circuitry of the rotary steerable drilling system for the purpose of controlling the downward mechanical force, i.e., weight of the drill bit against the formation being drilled. The dump or bypass valves of the mud motor and thruster are thus both independently controlled downhole by the control system of the rotary steerable drilling tool responsive to feedback signals from various sensors and can be selectively controlled by telemetry from the surface as well. This invention also concerns an actively controlled rotary steerable drilling system incorporating a turbine powered electric motor drive mechanism for geostationary positioning of a drill bit during its rotation by the rotary drill string, mud motor, or both and having the capability for selective employment of the electric motor as a brake when the torque of the bit/formation interaction is prevalent as compared to internal friction.
2. Description of the Related Art
An oil or gas well often has a subsurface section that is drilled directionally, i.e., inclined at an angle with respect to the vertical and with the inclination having a particular compass heading or azimuth. Although wells having deviated sections may be drilled at any desired location, such as for "horizontal" borehole orientation or deviated branch bores from a primary borehole, for example, a significant number of deviated wells are drilled in the marine environment. In such case, a number of deviated wells are drilled from a single offshore production platform in a manner such that the bottoms of the boreholes are distributed over a large area of a producing horizon over which the platform is typically centrally located and wellheads for each of the wells are located on the platform structure.
Whether well drilling is being done on land or in a marine environment, there exists a present need in well drilling activities for extended reach drilling, which is accomplished according to the teachings of the present invention by achieving better transfer of weight and torque to the drill bit during drilling operations. High performance/power drilling is also achieved by the present invention by causing good transfer of weight and torque to the drill bit being controlled by the rotary steerable drilling system set forth in detail below. In circumstances where the well being drilled is of complex trajectory, the capability provided by the rotary steerable drilling system of this invention to steer the drill bit while the drill bit is being rotated by the collar of the tool enables drilling personnel to readily navigate the wellbore from one subsurface oil reservoir to another. The rotary steerable drilling tool enables steering of the wellbore both from the standpoint of inclination and from the standpoint of azimuth so that two or more subsurface zones of interest can be controllably intersected by the wellbore being drilled.
A typical procedure for drilling a directional borehole is to remove the drill string and drill bit by which the initial, vertical section of the well was drilled using conventional rotary drilling techniques, and run in at the lower end of the drill string a mud motor having a bent housing which drives the bit in response to circulation of drilling fluid. The bent housing provides a bend angle such that the axis below the bend point, which corresponds to the rotation axis of the bit, has a "toolface" angle with respect to a reference, as viewed from above. The toolface angle, or simply "toolface", establishes the azimuth or compass heading at which the deviated borehole section will be drilled as the mud motor is operated. After the toolface has been established by slowly rotating the drill string and observing the output of various orientation devices, the mud motor and drill bit are lowered, with the drill string non-rotatable to maintain the selected toolface, and the drilling fluid pumps, "mud pumps", are energized to develop fluid flow through the drill string and mud motor, thereby imparting rotary motion to the mud motor output shaft and the drill bit that is fixed thereto. The presence of the bend angle causes the bit to drill on a curve until a desired borehole inclination has been established. To drill a borehole section along the desired inclination and azimuth, the drill string is then rotated so that its rotation is superimposed over that of the mud motor output shaft, which causes the bend section to merely orbit around the axis of the borehole so that the drill bit drills straight ahead at whatever inclination and azimuth have been established. If desired, the same directional drilling techniques can be used as the maximum depth of the wellbore is approached to curve the wellbore to horizontal and then extend it horizontally into or through the production zone. Measurement-while-drilling "MWD" systems commonly are included in the drill string above the mud motor to monitor the progress of the borehole being drilled so that corrective measures can be instituted if the various borehole parameters indicate variance from the projected plan.
Various problems can arise when sections of the well are being drilled with the drill string non-rotatable and with a mud motor being operated by drilling fluid flow. The reactive torque caused by operation of a mud motor can cause the toolface to gradually change so that the borehole is not being deepened at the desired azimuth. If not corrected, the wellbore may extend to a point that is too close to another wellbore, the wellbore may miss the desired "subsurface target", or the wellbore may simply be of excessive length due to "wandering". These undesirable factors can cause the drilling costs of the wellbore to be excessive and can decrease the drainage efficiency of fluid production from a subsurface formation of interest. Moreover, a non-rotating drill string may cause increased frictional drag so that there is less control over the "weight on bit" and the rate of drill bit penetration can decrease, which can result in substantially increased drilling costs. Of course, a non-rotating drill string is more likely to get stuck in the wellbore than a rotating one, particularly where the drill string extends through a permeable zone that causes significant build up of mud cake on the borehole wall.
Two patents of interest to the subject matter of the present invention are U.S. Pat. Nos. 5,113,953 and 5,265,682. The '953 patent presents a directional drilling apparatus and method in which the drill bit is coupled to the lower end of a drill string through a universal joint, and the bit shaft is pivotally rotated within the steerable drilling tool collar at a speed which is equal and opposite to the rotational speed of the drill string. The present invention is significantly advanced as compared to the subject matter of the '953 patent in that the angle of the bit shaft or mandrel relative to the drill collar of the present invention is variable rather than being fixed. Additionally, the provision of a braking system (electrical, mechanical or hydraulic) in the rotary steerable drilling tool of the present invention is another significant advance over the teachings of the prior art. Even further, the presence of various position measurement systems and position signal responsive control in the rotary steerable drilling system of the present invention distinguishes it from the prior art. The present invention is also distinguished from the teachings of the prior art in the assembly of drilling system controllable mud motor and thruster apparatus and a flexible sub that can be arranged in any suitable assembly to enable directionally controlled drilling to be selectively powered by the rotary drill string, the mud motor, or both, and to provide for precision control of weight on bit and accuracy of drill bit orientation during drilling.
The '682 patent presents a system for maintaining a downhole instrumentation package in a roll stabilized orientation by means of an impeller. The roll stabilized instrumentation is used for modulating fluid pressure to a set of radial pistons which are sequentially activated to urge the bit in a desired direction. The drill bit steering system of the '682 patent most notably differs from the concept of the present invention in the different means that is utilized for deviating the drill bit in the desired direction. Namely, the '682 patent describes a mechanism which uses pistons to force the bit in a desired lateral direction within the borehole. In contrast, the rotary steerable drilling system of the present invention keeps the drill bit pointing in a desired borehole direction, despite rotation of the drill collar, by utilizing an impeller to drive an alternator, the output of which drives an electric motor to rotate the bit shaft axis about a universal joint at the same rotational frequency as the bit shaft is driven in rotary manner by the tool collar. The rotary steerable drilling system of the present invention also utilizes a braking system (electrical, hydraulic or mechanical) to control the rotation of the bit shaft when the torque of the bit/formation interaction is prevalent as compared to internal friction. Within the scope of the present invention the sensors and electronics of the tool may be rotated along with the drilling tool collar or may be maintained geostationary along with the axis of the bit shaft of the rotary steerable drilling system.